1. Field of the Invention
The present invention relates to wireless terminal devices and, more particularly to a wireless terminal device, such as a portable telephone, having a receiving unit with a direct conversion system in which a received signal is converted directly into a base band signal.
2. Description of the Background Art
In a heterodyne conversion system for receivers, which has been well known, a received signal is converted into an intermediate frequency (IF) signal by a mixer (MIX) and then an unwanted wave is removed by a band-pass filter (BPF) for the IF. After that, complex envelope detection is performed by a quadrature mixer (QMIX) to supply a base band signal. In the heterodyne conversion system, however, as the intermediate frequency exists an intermediate frequency circuit, such as a band-pass filter, must be provided, resulting in the increase in the number of components and space for mounting the components.
On the other hand, in a direct conversion system, a received signal is directly converted into a base band signal by a quadrature mixer. FIG. 1 is a block diagram referenced for describing the direct conversion system.
FIG. 1 shows an example in which the direct conversion system is employed for a portable telephone which operates according to a certain communication system, such as CDMA system, in which transmission and reception are performed simultaneously and, a transmission system connected to a duplexer (DUP) 2 is not shown. In FIG. 1, a high-frequency signal received by an antenna 1 is supplied to a low-noise amplifier (AMP) 3 via duplexer 2, a received signal of a low level is amplified, an unwanted high-frequency band component such as an external interference wave is removed by a band-pass filter (BPF) 4 and the remaining component is converted into a base band signal by a quadrature mixer (QMIX) 5.
Here, the relation between a frequency frx of the received signal, which is an output from band-pass filter 4 and a local oscillation frequency flo of quadrature mixer 5 is represented by the following expression:
fbb=frxxe2x88x92flo,
where fbb is the base band signal. As the frequency of the base band signal is sufficiently lower than that of the received signal, frx=flo holds.
The base band signal obtained through conversion is amplified by a low-noise amplifier (AMP) 6 and the resulting signal is supplied to a low-pass filter (LPF) 7 where an interference wave such as an adjacent channel component is removed from the base band signal.
Further, the base band signal is supplied to a variable gain amplifier (VGA) 8 and amplified to maintain an input to an A/D converter 9 in the next stage at a constant level and, the resulting signal is converted into a digital signal by AID converter 9 and, demodulated at a demodulation circuit (DEM) 10. To achieve this operation, the level of the signal supplied to A/D converter 9 must not become saturated at A/D converter 9 or exceed a level of resolution.
In the receiver of the direct conversion system shown in FIG. 1, the elimination of the mixer and the band-pass filter for the intermediate frequency required in the heterodyne conversion system is allowed to achieve a simple circuit structure. In addition, a spurious response of the receiver to a signal represented by an image signal can be mitigated whereby the high-frequency band-pass filter can be replaced with a smaller band-pass filter 4.
In the portable telephone, however, the level of the base band signal supplied to A/D converter 9 at the onset of the operation, such as at power-on, is not predictable. To deal with this problem, the gain of variable gain amplifier 8 can be set to a suitable value based on an assumption.
When the initial set value of the gain of variable gain amplifier 8 is high, however, if a signal of a higher level than an input voltage range of A/D converter 9 as shown in FIG. 2A is supplied and is amplified with a high amplification factor by variable gain amplifier 8 and input to A/D converter 9, A/D converter 9 is saturated and the detection of the signal becomes impossible. In this case, a suitable level of the gain of variable gain amplifier 8 to be set for the next step cannot be known.
Conversely, when the initial set value of the gain of variable gain amplifier 8 is low, if a signal of a lower level than the resolution of A/D converter 9 as shown in FIG. 2B is supplied, the signal is hardly amplified by variable gain amplifier 8 and, the signal detection is not possible with the resolution of A/D converter 9. Hence, a suitable level of the gain of variable gain amplifier 8 to be set for the next step cannot be known.
Therefore, it is necessary to predict the suitable gain of variable gain amplifier 8, and to change the setting of the value a few times until the suitable value is set. Thus, the setting of the gain of variable gain amplifier 8 to an optimum level takes time.
To solve the above-described problem, the input voltage range of A/D converter 9 may be widened and A/D converter 9 with fine resolution may be employed. Such A/D converters, however, consume large current and are large in circuit size and their use in portable telephones is subjected to a certain limitation.
Thus, a main object of the present invention is to provide a wireless terminal device allowing a swift and secure gain control of the variable gain amplifier at the start of the operation.
The present invention is a wireless terminal device wherein a received signal is supplied to a variable gain circuit having a variable gain, a level of the received signal supplied to the variable gain circuit is detected by a first level detection circuit and, an input voltage range of the first level detection circuit is selected wide. An output signal of the variable gain circuit is converted into a digital signal by a conversion circuit, a level of an input signal or an output signal of the conversion circuit is detected by a second level detection circuit and, at the start of an operation, a control circuit coarsely controls the gain of the variable gain circuit based on a detection output from the first level detection circuit and thereafter finely controls the gain of the variable gain circuit based on a detection output from the second level detection circuit, thus the control is performed such that a signal supplied to the conversion circuit is in an input voltage range of the conversion circuit and detectable.
According to another aspect of the invention, the first level detection circuit is a logarithmic amplifier. The control circuit supplies power to the logarithmic amplifier at the start of the operation, performs a feedforward control such that the gain of the variable gain circuit is controlled based on a detection level of the logarithmic amplifier and then performs a feedback control such that the gain of the variable gain circuit is controlled based on the detection output of the second detection circuit. Thereafter, the control circuit stops power supply to the logarithmic amplifier.
According to still another aspect of the invention, a mobile terminal device is provided performing a level detection at predetermined intervals such that when the wireless terminal device moves to an adjacent wireless area, a reception level of a signal from the adjacent wireless area can be detected. The control circuit supplies power to the logarithmic amplifier shortly before the predetermined interval is elapsed.
According to a further aspect of the invention, an amplification circuit is provided in a previous stage to the variable gain circuit. The control circuit sets a gain of the amplification circuit and an input voltage range of the first level detection circuit based on a level detected by the first level detection circuit when the gain of the amplification circuit is set to a maximum value or a minimum value at the start of the operation and a level detected by a first level detection circuit when the gain of the amplification circuit is set to a minimum value or a maximum value.
According to a still further aspect, a quadrature frequency conversion circuit is provided in a previous stage to the variable gain circuit. The high-frequency input signal is converted into two signals with the same level and different phases, and the variable gain circuit and the conversion circuit are provided corresponding to two signals with different phases, respectively. The first level detection circuit detects and supplies a level of one of two signals with different phases to the control circuit. The second level detection circuit detects root-mean square values of respective detection signals of two variable gain circuits and supplies the values to the control circuit.
According to a still further aspect of the invention, a wireless terminal device directly converting the high-frequency signal into the base band signal is provided. The base band signal is supplied to a variable gain circuit having a variable gain. A level of the base band signal supplied to the variable gain circuit is detected by the level detection circuit. The input voltage range of the level detection circuit is selected wide. The output signal of the variable gain circuit is converted into a digital signal. The control circuit controls the gain of the variable gain circuit based on the detection output of the level detection circuit at the start of the operation and thus the adjustment is performed such that the signal supplied to the conversion circuit is within the input voltage range of the conversion circuit and detectable with the resolution thereof.